A newly detected parasite in a major recreational freshwater site in Portugal is putting scientists and local communities on alert. The first-of-its-kind finding in Portugal suggests a little-known health risk may already be present in Western Europe's largest artificial reservoir.
SciTechDaily detailed a study published in Science in One Health that provided evidence of Trichobilharzia parasites, including T. franki, in Lake Alqueva. The findings suggest the ailment, known as swimmer's itch according to the CDC, may already be present, potentially affecting people who come into contact with contaminated water at the popular site used for swimming, fishing, and birdwatching.
The parasitic flatworms attach themselves to waterfowl and snails before inadvertently penetrating human skin, per SciTechDaily. Cercarial dermatitis, the proper name for swimmer's itch, is generally mild, but it can still bring severe itching, bumps, and lesions, while repeated exposure may make symptoms worse. T. franki is one of the species most often associated with the condition.
To uncover whether the parasites were present on the lake, researchers gathered 7,125 freshwater snails across eight field visits and 25 sites between May 2023 and October 2024. Molecular analysis was possible for 325 of those snails. After testing, four snails were shedding larvae consistent with Trichobilharzia. The infected snails were all traced to a single site, Campinho, during the September 2024 period.
While these numbers seem low, the results were highly localized and specific to one type of snail. That suggests the Campinho site was a hotspot, which is further backed up by reports of swimmer's itch in the area, per the study.
There is also a bigger ecosystem story at play here. Genetic analysis linked the Portuguese samples closely to strains reported in Austria and Hungary, according to the study. Researchers theorized that migratory birds might be spreading the parasite all the way from those countries. They spotted numerous waterfowl that could fit the bill in the neighborhood of the lake.
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For residents and tourists, Trichobilharzia parasites' unwanted presence can be more than a nuisance. As SciTechDaily noted, Lake Alqueva peaks in the summer months when human water contact and snail activity are at their highest levels. Swimmer's itch isn't the most concerning ailment, but if it causes visitors to stay out of the water or is pervasive enough to close down beaches, that can have an impact on tourism.
In the study, the scientists suggest that more research is needed to understand just how prevalent the parasites are and what is causing them to spread. They suggest looking to the animals hosting them to better control and manage the fallout.
"Monitoring bird populations and snail habitats can help mitigate CD risk, allowing timely public-health actions and local warnings when needed," they concluded, per SciTechDaily. "Public health efforts should also prioritize awareness campaigns on CD and expand surveillance to other high-risk freshwater systems in Portugal."
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